The Nguyen family and the Cooper family each used their sprinklers last summer. The water output rate for the Nguyen family's sprinkler was 30L per hour. The water output rate for the Cooper family's sprinkler was 35L per hour. The families used their sprinklers for a combined total of 40 hours, resulting in a total water output of 1325L. How long was each sprinkler used?

Assume that there is a 11% rate of disk drive failure in a year. a. if all your computer data is stored on a hard disk drive with a copy stored on a second hard disk drive, what is the probability that during a year, you can avoid catastrophe with at least one working drive? b. if copies of all your computer data are stored on four independent hard disk drives, what is the probability that during a year, you can avoid catastrophe with at least one working drive?